Tech Briefs

Reducing process steps associated with hybridized structures could reduce manufacturing costs.

Lightweight composite structures offer significant long-term cost-saving opportunities for the U.S. Army as replacements for traditional metal structures. However, high-strength economical bonding processes must be developed to join composites in order for them to be viable replacements. The Army is exploring approaches, also under evaluation in the commercial aerospace industry, to reduce the process steps associated with composite bonding.

Schematic of the layup and bagging sequence for the VARTM process.

A potential single-step process for preparing composite surfaces uses removable fabrics known as peel plies, which are cured to the composite surface during fabrication. Traditionally, peel plies have not been a reliable media for creating optimal bonding surfaces. However, pre-impregnated versions, known as wet peel plies, show significantly improved potential, though at a greater cost compared to their traditional dry fabric counterparts, in creating excellent composite bonding surfaces. Still, reducing the processing steps would reduce overall cost and shorten production lead times of composite systems for the Army in applications such as composite armor and structural vehicle components.

Typically, dry peel ply fabrics have been used as a protective layer for the composite surface after fabrication. The peel ply is removed prior to surface preparation methods for secondary bonding, which include mechanical abrasion, chemical treatments, cleaning, and inspection. The success or failure of a peel ply to create a bondable surface is defined by its ability to consistently provide clean and highly chemically active substrates.

Successful removal of a peel ply requires fracture within the resin matrix between the peel ply and reinforcement fibers, and complete removal of the peel ply fabric without residual fiber transfer. Undesirable modes of removal include interfacial fracture between the peel ply and the matrix, peel ply fracture, and inter-laminar failure within the composite. Transfer of release agents used to facilitate their removal has been widely shown to have significant negative influence on bond performance. Virtually all previous studies that focus on the effects of peel plies on composite bonding conclude that there is no universally acceptable peel ply.

Most current commercially available peel plies are dry fabrics with or without a release coating applied. Typical yarn types are nylon, polyester, and fiberglass in various weave styles. Common release coatings include silicone, polyamide, and polytetrafluoroethylene (PTFE). Non-coated fabrics are typically polyester because it is highly inert. Several fabric characteristics reported to be critical to successful release include yarn type, weave style, and release coating. However, previous studies show varying conclusions regarding the influence of peel ply characteristics on bond performance.

This work examines surface preparation methods for secondary bonding of composite substrates using FM® 94K (Cytec, Havre de Grace, MD) epoxy film adhesive. Laminates using the U.S Army legacy composite system consisting of plain-woven S2 fiberglass (BGF, Greensboro, NC) and SC-15 epoxy resin (Applied Poleramic Inc., Benicia, CA) were prepared by the Vacuum Assisted Resin Transfer Molding (VARTM) process. Two methods were used for tool-side surface preparation: various commercial off-the-shelf (COTS) peel plies, and peel ply in combination with grit blast-prepared surfaces. Nonprepared virgin surfaces and grit blast only-prepared surfaces were also compared.

This work was done by Jared M. Gardner, James P. Wolbert, Larry R. Holmes, Jr., and Daphne D. Pappas for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Materials category . ARL-0216